Low loss composition of BaxSryCa1-x-yTiO3: Ba0.12-0.25Sr0.35-0.47Ca0.32-0.53TiO3

ABSTRACT

A dielectric thin-film material for microwave applications, including use as a capacitor, the thin-film comprising a composition of barium strontium calcium and titanium of perovskite type (Ba x Sr y Ca 1−x−y )TiO 3 . Also provided is a method for making a dielectric thin film of that formula over a wide compositional range through a single deposition process.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 09/420,873, filedOct. 19, 1999, now U.S. Pat. No. 6,146,907.

GOVERNMENT RIGHTS

The United States Government has a paid-up license in this inventionpursuant to Contract No. DE-AC03-76SF00098 between the United StatesDepartment of Energy and the University of California for management ofthe Lawrence Berkeley National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

The invention generally relates to dielectric thin-films belonging tothe Ba_(x)SryCa_(1−x−y)TiO₃ class of perovskite oxide and fabricationmethods thereof.

2. Background Art

Thin-film ferroelectric materials have application for variouselectronic devices, such as dynamic and ferroelectric random accessmemories (“DRAM” and “FRAM”). Thin-film ferroelectric materials are alsowidely used in the development of new microwave devices such asfrequency agile filters, phase shifters, and tunable high-Q resonatorsas taught generally in L. A. Knauss, et.al., Appl. Phys. Lett. 69:25-27(1996) and P. Bhattacharya, et.al., Jpn. J. Appl. Phys., 32:4103-4106(1993).

The dielectric currently utilized in DRAMS and for microwaveapplications is SiO₂ or a silicon oxide/nitride composite layer (“ONO”)with a relative dielectric constant of 6. However, as integrated circuitdevices move toward higher and higher integration densities, severedemands are placed on the device design, particularly with respect tosqueezing storing capacity into a smaller cell space. A capacitance ofabout 9 fF/μm² appears to be the maximum achievable value for ONO typeof materials. As a result, since the mid-1980's, there has been anincreasing effort to replace the ONO dielectric with an alternativedielectric having a substantially higher capacitance per unit area.

Most attention has been focused on (Ba_(x)Sr_(1−x))TiO₃ (“BST”), asthese materials possess high dielectric constants (∈_(r)) and low loss(tan δ). At room temperature, single crystal SrTiO₃ has a very low loss(tan δ<10⁻⁴) but also a low dielectric constant. On the other hand,BaTiO₃ has very high dielectric constants but high loss. Mixing Sr andBa has resulted in BST materials with high dielectric constants andimproved tan δ over BaTiO₃. BST is a ferroelectric with the perovskitestructure. The BST solid solution also shifts the Curie point of BaTi0₃at 130° C. to around room temperature for Ba_(0.7)Sr_(0.3)TiO₃, thusachieving the maximum permittivity around the operating temperature.

A method of making various elemental compositions of BST type materialsis taught by Azuma, et.al., in U.S. Pat. No. 5,723,361. Azuma usesmolecular precursors, preferably metal carboxylates or metal alkoxidesdissolved in an organic solvent such as xylene. The thoroughly mixedsolution is then coated on a substrate by a “spin-on” depositionprocess. Following each spin coat the solvent is removed by a lowtemperature drying process. The desired thickness of the final film thusdepends upon the number of spin-dry cycles in the process. AlthoughAzuma suggests the combination of the metal calcium with the core BSTmaterial the method of Alzuma requires the spin coating of a liquidmetal precursor, preferably in the form of a metal carboxylate oralkoxide. Also, the traditional co-deposition method of Alzuma permitsonly one specific material composition to be made per depositionprocess. That is, the final stoichiometry of the BST type material isuniform throughout the deposited material, and is predetermined by thesolution of mixed metal concentrations. In the present invention, theunique deposition process results in a BST type material that isnon-uniform throughout the deposited material, and thus allows thetesting of various metal compositions from a single deposition process.

SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)

The present invention is of a dielectric thin film comprising barium,strontium, calcium, titanium and oxygen of general formula(Ba_(x)Sr_(y)Ca_(1−x−y))TiO₃. In the preferred embodiment the thin filmcomprises the following mole fraction of Group IIA metals; barium isapproximately about 0.12 to 0.25, strontium is approximately about 0.35to 0.47, and calcium is approximately about 0.32 to 53. A dielectricthin film with such an elemental composition would typically have adielectric constant between approximately 130 and 160 and tan δ of lessthan 0.02.

The present invention is also of a method of forming a dielectric thinfilm of general formula (Ba_(x)Sr_(y)Ca_(1−x−y))TiO₃ comprising:providing a substrate, depositing titanium dioxide, barium, strontium,and calcium on the substrate, and heating the substrate containing thedeposited titanium dioxide, barium, strontium, and calcium. Thedepositing of titanium dioxide, barium, strontium, and calcium can beaccomplished in a sequentially random order or concurrently. In thepreferred embodiment, the titanium dioxide, barium, strontium, andcalcium are deposited by pulse laser deposition.

The present invention is also of a method of forming a dielectric thinfilm of general formula (Ba_(x)Sr_(y)Ca_(1−x−y))TiO₃ comprising:providing a substrate, depositing TiO₂, BaCO₃, SrCO₃ and CaCO₃on thesubstrate, and heating the substrate containing the deposited TiO₂,BaCO₃, SrCO₃ and CaCO₃. The depositing of TiO₂, BaCO₃, SrCO₃ and CaCO₃can be accomplished in a sequentially random order or concurrently. Inthe preferred embodiment, the TiO₂, BaCO₃, SrCO₃ and CaCO₃ are depositedby pulse laser deposition. Following the deposition of the variouscarbonates the deposited film is heated between approximately about 200and 500° C., preferably 400° C., for at least approximately 24 hours,followed by a heating step of approximately 500 and 800° C. for at leastapproximately 12 hours, preferably at 500, 600, 700 and 800° C. for atleast approximately 3 hours at each temperature, and finally heatingbetween approximately 900 and 950° C. for at least approximately fourhours, preferably at 900 and 950° C. for at least approximately 2 hoursat each temperature.

The present invention is also of a method of forming a dielectric thinfilm of general formula (Ba_(x)Sr_(y)Ca_(1−x−y))TiO₃ comprising:providing a triangular shaped substrate, depositing titanium dioxide,barium, strontium, and calcium, preferably TiO₂, BaCO₃, SrCO₃ and CaCO₃,on the triangular shaped substrate in a sequentially random order with acomputer controlled shutter system, rotating the triangular shapedsubstrate 120° after each subsequent deposition, and heating thetriangular shaped substrate containing the deposited titanium dioxide,barium, strontium, and calcium. In the preferred embodiment, thetitanium dioxide, barium, strontium, and calcium, preferably TiO₂,BaCO₃, SrCO₃ and CaCO₃, are deposited by pulse laser deposition.Following the deposition of the various carbonates the deposited film isheated between approximately about 200 and 500° C., preferably 400° C.,for at least approximately 24 hours, followed by a heating step ofapproximately 500 and 800° C. for at least approximately 12 hours,preferably at 500, 600, 700 and 800° C. for at least approximately 3hours at each temperature, and finally heating between approximately 900and 950° C. for at least approximately four hours, preferably at 900 and950° C. for at least approximately 2 hours at each temperature.

The present invention is also of a dielectric thin film manufactured bythe methods described above.

The present invention solves the problem of having to make numerousindividually distinct compositional specimens by generating acompositional spread of compounds through a single deposition process.Thus, the invention provides the best way to rapidly survey a largerange of select material compositions with optimal electronic ordielectric properties. To generate such a compositional spread, thedesired metal precursors are deposited sequentially using a highprecision computer controlled shutter. Following an annealing step thistechnique generates a precisely controlled stoichiometric profile withina very small area compared to the traditional co-deposition processes.

A primary object of the present invention is the method of making athin-film, ferroelectric material of non-uniform composition,Ba_(x)Sr_(y)Ca_(1−x−y)TiO₃ (“BSCT”) through a single deposition process.This process permits the testing of specific BST material compositionsfor low dielectric loss or leakage and an acceptable dielectric constantwithout the need to individually make each individual compositionalspecimen.

A further object of the present invention is to provide a thin-film BSCTwith low dielectric loss or leakage and an acceptable dielectricconstant, and having enhanced application, particularly forhigh-frequency or microwave applications, for uses such as DRAM,frequency agile filter, phase shifter, and tunable high-Q resonatordevices.

A further object of the present invention is to provide a method forfabrication of thin-film BSCT with low dielectric loss or leakage, andan acceptable dielectric constant.

A primary advantage of the invention is to provide a method to make awide compositional range of dielectric BST type materials that have lowloss (tan δ) and high dielectric constants through a singlecombinatorial deposition process rather than the need to fabricate eachnovel material composition separately.

Another advantage of the present invention is that the low dielectricloss or leakage of thin-film BSCT allows for higher integrationdensities. This in turn permits the manufacture of smaller devices,including memory devices such as DRAM devices, and devices such asfrequency agile filters, phase shifters, and tunable high-Q resonators.

Yet another advantage of the present invention is that the lowdielectric loss or leakage of thin-film BSCT permit the manufacture ofdevices with a lower power consumption requirement, including memorydevices such as DRAM devices, and devices such as frequency agilefilters, phase shifters, and tunable high-Q resonators.

Other objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, taken in conjunction with theaccompanying drawings, and in part will become apparent to those skilledin the art upon examination of the following, or may be learned bypractice of the invention. The objects and advantages of the inventionmay be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate several embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating a preferred embodiment of the invention and are not to beconstrued as limiting the invention. In the drawings:

FIG. 1a is a schematic of a deposition system with a computer-controlledshutter used to generate a linear gradient of precursors, the shutterbeing placed about 20 microns above the substrate on which films aredeposited;

FIG. 1b is a profile of resultant pre-annealed precursor layers ofCaCO₃, SrCO₃, and BaCO₃ (viewed edge on and with respect to FIG. 1a);and

FIG. 2 is the Θ/2Θ XRD pattern of a Ba_(0.2)Sr_(0.4)Ca_(0.4)Tisynthesized from layers of BaCO₃, SrCO₃, CaCO₃ and TiO₂ on (001) LaAlO₃.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Best Modes for Carrying Outthe Invention)

In the integrated circuit art, an acceptable crystalline material isoften referred to as a “substrate.” Throughout the specification andclaims, the term “substrate” defines any specified layer or crystalstructure, and most generally refers to any support for another layer.This may include any of a variety of silicon crystals, and may alsoinclude LaAlO₃ and MgO substrates, among others. The term “BSCT”includes materials of the general form (Ba_(x)Sr_(y)Ca_(1−x−y))TiO₃.

The crystalline BSCT composition of the invention preferably has aperovskite type structure as will be understood by those skilled in theart. These BSCT materials are classified as ferroelectrics, andtypically exhibit ferroelectricity at room temperature. The inventedmaterials typically have high dielectric constants and relatively lowdielectric losses, and are useful as high dielectric constantcapacitors, whether or not they exhibit ferroelectric properties atnormal operating temperatures.

In a material having the form (Ba_(x)Sr_(y)Ca_(1−x−y))TiO₃, the ratio ofBa to Sr to Ca is variable but the total number of Ba, Sr, and Ca atomsis fixed with respect to Ti atoms and O atoms. This ratio is expressedgenerally as (Ba_(x)Sr_(y)Ca_(1−x−y))TiO₃ indicating that the total ofBa, Sr and Ca atoms equals one (x+y+1−x−y=1) combined with one Ti atomand three O atoms. As a result, the amounts of each Group IIA cationstated in a particular compositional mixture is in terms of atomic ormole fractions of the total number of atoms or moles of Group IIA metalcations.

While the preferred method of depositing Group IIA metal precursors isby pulsed laser deposition, any means for depositing Group IIA metalprecursors known to the art may be employed, such as sputtering,chemical vapor deposition CVD, and metallorganic CVD. In one preferredembodiment, a compositional spread of BaCO₃, SrCO₃, and CaCO₃ is layeredat room temperature with a pulsed laser deposition system equipped withan in situ shutter system, as shown schematically in FIG. 1a. Theshutter is computer controlled and moves parallel to the surface of thetriangular substrate during the course of a deposition at various andpredetermined rates to give a linear thickness gradient (or arbitrarylinear profile) of the layered carbonate precursors. This techniquegenerates precisely controlled compositional profiles within a verysmall area. Post annealing of the layered carbonate materials results inhigh quality epitaxial BSCT films.

Fabrication of the thin-film ferroelectric BSCT materials, begins withthe deposition of TiO₂ to a uniform, suitable thickness, such as 750 Å,on a triangular, single-crystal LaAlO₃ substrate. High vacuum pulsedlaser deposition coupled with scanning of laser beam across Group IIAtargets results in the layering of the Group IIA carbonates onto theTiO₂ layer. Alternatively, a thin-film BSCT can be formed using aplurality of concurrently sputtered Group IIA and Ti targets. Moving theshutter during the deposition as illustrated in FIG. 1a generated alinear thickness gradient of CaCO₃. Gradients of SrCO₃ and BaCO₃ werethen deposited in the same way by rotation of the substrate 120°. Theresultant thin film profile along the dotted line of the triangularsubstrate in FIG. 1a is shown in FIG. 1b.

It is preferred to first anneal the layered carbonate film attemperatures of 200 to 500° C., preferably 400° C., for at least 24hours to allow homogeneous mixing of precursors into an amorphousintermediate before crystallization at higher temperatures. Also,because the carbonates are moisture sensitive it is preferred tomaintain a temperature of at least 200° C. during the time periodbetween deposition of the carbonates and the annealing process.Following the 24 hour anneal period, the substrate containing thedeposited titanium dioxide, barium, strontium, and calcium is preferablyheated between approximately 500 to 800° C. for at least 12 hours,preferably at approximately 500, 600, 700 and 800° C. for 3 hours ateach temperature. The substrate containing the deposited titaniumdioxide, barium, strontium, and calcium is further heated betweenapproximately 900 and 950° C. for about 4 hours, preferably atapproximately 900 and 950° C. for about 2 hours at each temperature.This procedure allows one to synthesize thin films of all compositionswith the composition spread of Ba_(x)Sr_(y)Ca_(1−x−y)TiO₃.

The microwave dielectric properties of the BSCT material at 1 GHz wasanalyzed with a scanning Evanescent Microwave Microscope (SEMM).Compositions of Ba_(0.12-0.25)Sr_(0.35-0.47)Ca_(0.32-0.53)TiO₃ had thelowest tan δ and are the best candidates for microwave dielectricapplications. These films have tan δ less than 0.02 and ∈_(r)'s valuesbetween 130-160.

Although the invention has been described in detail with particularreference to these preferred embodiments, other embodiments can achievethe same results. Variations and modifications of the present inventionwill be obvious to those skilled in the art and it is intended to coverin the appended claims all such modifications and equivalents. Theentire disclosures of all references, applications, patents, andpublications cited above are hereby incorporated by reference.

What is claimed is:
 1. A dielectric thin film for microwave applicationscomprising barium, strontium, calcium, titanium and oxygen of generalformula (Ba_(x)Sr_(y)Ca_(1−x−y))TiO₃ wherein the mole fraction of bariumis approximately 0.12 to 0.25, strontium is approximately 0.35 to 0.47,and calcium is approximately 0.32 to 0.53, said film having a dielectricloss, tan δ, less than 0.02, and dielectric constant, ∈_(r), between130-160.